Positron emission tomography (PET) is used for in vivo longitudinal studies of potential therapies in animal models of human disease. Small animal PET moreover permits studies of fundamental biological processes that are not possible with other imaging modalities. With the advent of high resolution dedicated small animal scanners the field has grown rapidly in the last few years. Small animal PET has been used in a wide variety of studies at UW-Madison. UW is home to macaque colonies at the Harlow Center for Biological Psychology, the Wiconsin National Primate Research Center, and the School of Medicine and Public Health, and rhesus brain PET studies performed at Madison in the early 1990s on the ECAT 931 human scanner are among the earliest in the literature. In August 2002 UW acquired a microPET P4, which has been used to acquire 1859 animal scans, including 1515 rhesus brain scans (as of 12/31/2009). Funding is requested for a microPET Focus 220 small animal positron emission tomography (PET) scanner built by Siemens Medical Solutions USA (Knoxville, TN). The Focus 220 provides improved spatial resolution (1.4 mm FWHM) and sensitivity (4%) along with a large field of view (19 cm transaxial and 7.6cm axial). This permits more detailed imaging of macaque brain with reduced tracer mass as well as simultaneous imaging of multiple rats or mice. In this resubmission, we note that the requested instrument is essential to ensure the continued capability to carry out NIH funded research. The vendor, Siemens, gave formal notification in July of 2009 that the end of support date for the P4 will be July 1, 2010 due to lack of availability of replacement components. This raises the possibility of losing use of the existing instrument in the near future. PUBLIC HEALTH RELEVANCE: Funding is requested for a small animal positron emission tomography (PET) scanner to replace existing equipment. This equipment will be used for longitudinal in vivo studies of animal models of human pathologies and imaging of fundamental biological processes. This will allow the UWMadison PET imaging program to continue to accommodate the increasing number of studies of pathologies such as mood disorders, fetal alcohol exposure, multiple sclerosis, and Parkinson's disease.